Prediction of the interfacial tension of sugar-based surfactants through molecular modelling Muhammad Ariif Hafiizhullah Kamrul Bahrin 1 , Harry Cárdenas 1 , S. Shahruddin 2 , Jofry B. Othman 2 , Andrés Mejía 3 , Omar K. Matar 1 , Erich A. Müller 1* 1 Imperial College London, United Kingdom, 2 Specialty Chemical Technology, PETRONAS Research Sdn. Malaysia, 3 Universidad de Concepción (UdeC), Chile Sugar-based surfactants (SBS) are amphiphiles with potential applications in various industrial settings. The large industrial-scale production of these “green” surfactants has become possible due to the availability of feedstocks synthesised from biomass through biorefineries [1] . As opposed to petrochemical surfactants, SBSs are carbon- neutral, non-toxic, potentially cost-effective, and can be suitably tailored by modifying their morphology [2] . A typical SBS is composed of a polar head derived from glucose, a linker and a hydrocarbon (alkane) tail (see Figure 1). There is, however, a large and diverse chemical ‘space’ to be explored if one is to design surfactants for specific applications, as small differences in the morphology of these components (e.g., length of tails) will affect the surfactant properties [3] . Experimental screening of candidate surfactants is costly and slow due to the prior need to synthesise and purify prototypes chemically. In silico calculations employing molecular dynamics (MD) simulations can rapidly assess interfacial properties over a wide range of operating conditions. A molecular modelling approach is presented by predicting the surface tension of non-ionic alkylpolyglucoside (APG 12 ) on the water/air interface. Canonical all-atom MD simulations are performed. The simulations consist of roughly 12000 molecules of water and a variable amount of surfactant molecules ranging from 25 to 170 APG 12 molecules. The air is represented by a vacuum in which specific water molecules can escape from the liquid phase at random. Each state point is run for a minimum of 12 ns at 298.15 K with a time-step of 1 fs using the pcff+ force field. Adsorption data obtained from the MD simulations, relating the interfacial tension, γ, (or surface pressure, Π) with surfactant surface coverage, Γ, are used to build a 2D simulation-based equation of state. The equation of state is used to derive a model for the surfactant excess chemical potentials at the interface. The adsorption isotherm of APG 12 (the γ – bulk concentration, c, curve) is determined from the derivation of a thermodynamic relationship that incorporates a free energy transfer. An enhanced sampling method is used to compute the free energy change associated with transferring a single surfactant molecule from the bulk solution phase to the interface, which is one of the inputs to the model. Figure 1 shows the predicted and the experimental interfacial tensions as a function of the bulk concentration, c. While experimental data (including ours)have uncertainties, it is seen that the simulation predictions are quantitatively correct. One notes that the pcff+ force field is parameterised to match bulk properties of organic moieties and, as such, was not specifically designed to reproduce interfacial properties accurately. A framework to predict adsorption isotherms of non-ionic surfactant solutions is presented, which is based solely on the chemical structures of the surfactant molecules and devoid of any empirical fitted parameters.
IP08
© The Author(s), 2023
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